CALIBRATION OF ACOUSTIC INSTRUMENTS FOR FISH DENSJ TY ESTIMATION: A PRACTICAL GUIDE

Transcription

1 CALIBRATION OF ACOUSTIC INSTRUMENTS FOR FISH DENSJ TY ESTIMATION: A PRACTICAL GUIDE by K. G. Foote, H. P. Knudsen and G. Vestnes Institute of Marine Research 5011 Bergen, Norway and D. N. MacLennari ande. J. Simmonds Marine Laboratory Victoria Road Aberdeen AB9 8DB, UK

2 COOPERATIVE RESEARCH REPORT No. 144 CALIBRATION OF ACOUSTIC INSTRUMENTS FOR FISH DENSITY ESTIMATION: A PRACTICAL GUIDE by K. G. Foote, H. P. Knudsen and G. Vestnes Institute of Marine Research 5011 Bergen, Norway and D. N. MacLennan and E. J. Simmonds Marine Laboratory Victoria Road Aberdeen AB9 8DB, UK e~o~ ~ess %~ O8 International Council for the Exploration of the Sea Pa1~gade 2~4, 1261 Copenhagen K Denmark February 1987

3 (i) CONTENTS Page SUMMARY (iv) LIST OF SYMBOLS (v) INTRODUCTION 1.1 The Application Scope of the Report Calibration Technique Organisation of the Report 3 2 THEORY AND DEFINITIONS On axis Sensitivity Time-Varied Gain Equivalent Beam Angle 6 3 ELEMENTS OF CALIBRATION On axis Sensitivity Introduction Example: stationary - sphere method Rigging Hydrography Centering Sphere range Echo integration SL+VR Example: moving - sphere method Method 13 Sphere range TVG correction Worked examples Time-Varied Gain Introduction Example: constant - output method Method Calculation of the TVG error function Worked example Example: measurement by special purpose unit The Time - Amplitude - Frequency (TAF) unit Measurment of the TVG deviation 3.a.3.3 with TAF 25 Worked example 26

4 (ii) Page 3.3 Equivalent Beam Angle Introduction Example: towed-body transducer Method Date collection Data processing Example: hull-mounted transducer Principle Materials Method Analysis Electrical Measurements Transmitter Power measurements with a voltage probe Example POwer measurements with a current probe Example Transducer Impedance measurement Example Receiver: total amplification Measurement procedure Example: measurement of attenuator Example: measurement of the total gain Attenuator setting Example: measurement of amplification at the attenuator setting 20 db (gain - 20 db) Echo-integrator Scaling Linearity Test measurement of linearity Dynamic range Adjustment of the QX~ preprocessor 45 4 CALIBRATION ACCURACY Time-Varied Gain Equivalent Beam Angle On-axis Sensitivity Measurement Summary of Errors 47 5 INTER-SHIP CALIBRATION Introduction Method Example 52

5 (iii) Page 6 CONCLUSIONS 54 7 ACKNOWLEDGEMENTS 54 8 REFERENCES 54 APPENDICES 57 I : Equations for Sound Speed and Absorption Coefficient 57 II Target Strengths of Calibration Spheres 59 Ill A Calibration Narrative 60 IV Calibration Worksheets 63 TABLES 1 Echo timing data for target range calculation 17 2 Data from TVG measurement 18 3 Estimation of on-axis sensitivity by moving the target through the transducer beam 19 4 Ideal TVG start time for exact range compensation 24 5 Theorectical voltage amplitude Vr at range r for the calibrated output signal of the EK38 echo sounder 27 6 Theoretical voltage amplitude Vr at range r for the calibrated output signal of the EK400/38 echo~-sounder 27 7 Components of calibration error 48 8 Summary of results from:the standard-sphere calibration 53 9 Instrument settings during the intercalibration experiment 53

6 (iv) SUMMARY The acoustic estimation of fish biomass, as currently practiced, requires calibrated equipment. A good calibration is essential to good biomass estimates although it cannot guarantee these. We describe how to perform a high-. precision calibration by means of a standard target sphere. This involves primary measurements of three quantities: (1) on axis sensitivity of the overall echo-sounding and integrating system, (2) time-varied-gain function of the receiver, and (3) equivalent beam angle of the transducer. If the transmitter, transducer, receiver or echo-integrator perform poorly, however, a good calibration, is not possible. Measurement of the electrical properties of the system components is therefore included as ai~i element of calibration. In this paper each of the four mentioned elements of! calibration are described both in principle and in practice, with detailed e~amples drawn from experience. Calibration accuracy and inter-ship calibration are also described in some detail. In conclusion, the provisional nature af this work is emphasized, as new developments will undoubtedly continue to impove on present techniques.

7 (v) LIST OF SYMBOLS A arbitrary constant b2g) beam pattern product; combined transmit and receive intensity in direction r c c(z) C C1 Cs speed of sound depth dependent function of speed of sound calibration factor the instrument constant constant in the equation for ~o, ~(R) dc~, ~ element of solid angle D ID1 E(R) Er E1 g G I M attenuation attenuator setting time-varied-gain (TVG) error for a target at range R average TVG error over depth channel of interest integral of the standard target echo, without TVG correction TVG correction factor total receiver amplification electrical current echo-integrator output from fish targets M1 echo-integrator output from the standard target, with TVG correctiou P average power inpu to the transducer R range of fish / Ra Rb upper limit of depth channel of interest lower limit of depth channel of interest R~ range corresponding to t~ R1 R2 range of standard target expiration range of TVG function

8 (vi) SL Sc source level on transmit transmitting current response S~ transmitting power response t tg th fime after start of transmit pulse time delay or start time for TVG function correction factor which allows for delay introduced by electronic filters and transducer response t1 time at which gain is measured t~ mid point of gate pulse used to sample receiver output t1 TS TS1 Ugen time from transmit pulse to rec~eived echo half-amplitude point target strength in db target strength of the standard target output voltage, of signal generator receiver input signal amplitude IJj~ Umax voltage on transducer terminals receiver output signal amplitude, at TVG expiration UZOm receiver output signal amplitude, at time corresponding to ZO m range Ur calibrated output signal amplitude peak to peak voltage of the transmitter output U1 V(R,t) sphere echo level receiver output amplitude from a target at range R VP. voltage response of transducer and echo sounder at expiration range of TVG function W~ TVG sample interval z z1 zz Z3 depth parameter depth of transducer depth of standard target transducer impedance at the centre frequency

9 (vii) acoustic absorption coefficient (measured in db/m) acoustic absorption coefficient (measured in nepers/m) ~(t) ~m(t) receiver~ voltage gain (amplitude) as a function of time ideal TVG function measured TVG function ~ CR) receiver voltage gain (amplitude) as a function of range ideal receiver voltage gain (amplitude) as a function of range corresponding to time t~ ~m,i(r) measured gain as a function of the nominal range R ~ c (t~ - tg) ~o, ~(R) measured gain function optimised to give g = 1 at range of fish p quantity of fish per unit area. The quantity may he either the number or the weight of fish. equivalent beam angle a backscattering cross section <a> average backscattering cross section of unit quantity of fish. To satisfy equation (1), the same units of fish quantity (number or weight) must be used in the definition of Pand <a>. effective backscattering cross section of standard target 0 beam angle of. circular transducer between 3 db down points 0A~ ~ 3 db angles of beam from a rectangular transducer

10 1 INTRODUCTION This report has been prepared as a result of discussions in the Fisheries Acoustics Science and Technology (FAST) working group of the ICES, particularly at meetings in Hirtshals, Denmark, 2-4 May 1984; Tromsj, Norway, May 1985; and Hull, England, May Acoustic instruments such as the echo-sounder have long been used in fisheries research. For more than two decades, there has been increasing interest in obtaining quantitative as well as qualitative information from fish echoes, especially for biomass estimation. This requires careful calibration of the acoustic equipment. Further, when more than one research ship is engaged on an acoustic survey, it is essential to ensure that calibrations are performed with sufficient accuracy by all concerned. 1.1 The Application Echo integration io the moot widely applied acouotic method for estimating the abundance of scattering organisms in the sea (Johannesson and Mitson, 1983; MacLennan and Forbes, 1984, 1986). The technique depends upon measuring the energy in the echoes received by an echo-sounder. The echoes are observed at the echo sounder output as electrical signals which are applied to the echo integrator. Thus the equipment performs an electrical measurement which then has to be converted to the estimate of fish density. 1.2 Scope of the Report According to the McGraw-Hill Dictionary of Scientific and Technical Terms, to calibrate means to determine by measurement or comparison with a standard the correct value of each scale reading on a meter or other device or the correct value for each setting of a control knob. In the case of the acoustic equipment used for fish stock surveys, the scale reading is the echo integrator output and the ~ correct value is the fish density in the transducer beam. The purpose of the calibration is to measure or deduce the conversion factors which relate the fish density to the echo-integrator output. To do this, we require knowledge of (a) the scattering properties of the targets which caused the echoes, normally described by th~ target strength, and (b) the performance of the equipment itself, such as the transducer sensitivity. Both (a) and (b) are relevant to the relationship between the fish density and the echo integrator output. However, the study of fish target strength is itself a major research topic and will not be considered here. In this report, we confine attention to the performance of echo-sounder and echo-integrator equipment, and how this performance should be measured. 1.3 Calibration Techniques Blue (1984) and Robinson (1984) have reviewed various techniques for measuring the performance of acoustic survey equipment. A brief summary is presented below.

11 2 The calibration may be performed as a single measurement of the complete acoustic system, as in (a) and (b) below, Including the fish target strength. However, this approach provides no information about the cause of observed variations which could be associated with equipment malfunction or changes in fish behaviour. The alternative is to perform separate measurements on component parts of the equipment, as in (c-e). These are techniques for determining the on-axis sensitivity. In addition, to complete the equipment calibration, two other parameters must be measured or estimated. These are the equivalent beam angle and the timevaried gain (TVG) function, which will be discussed in detail later in this report. (a) Measurements with caged ~ish (Johannesson and Losse, 1977). A known quantity of fish in a cage is insonified. Thus the calibration includes the fish target strength. However, the effect of captivity on the fish behaviour and hence the target strength is uncertain, so th~ results may not be representative of fish in the wild. Caged fish measureme~s are now considered to be too inaccurate for the calibration of fishery echo sounders. (b) The inter-ship calibration (MacLennan and Pope, 1983) is a direct comparison between the integrator outputs of two ships as they steam over the same area. This is a relative measurement and not an absolute calibration, unless one ship is regarded as a standard reference. The measurement may be subject to large fluctuations because of differences in fish density below the two ships, when the accuracy will be poor. The inter-ship calibration is inadequate by itself. Absolute calibration of the acoustic equipment is essential. (c) Reciprocity calibration (Robinson and Hood, 1983). This determines the combined source level and receiving sensitivity of the survey transducer by making acoustic measurements involving two other transducers. There are practical difficulties in aligning the transducers and achieving the necessary free field conditions which limit accuracy, especially when calibrating in less than ideal conditions at sea. (d) Calibrated hydrophone. This device is a secondary standard which is placed below the survey transducer to measure the source level. The accuracy depends upon the stability and alignment of the calibrated hydrophone and is generally very poor. Ce) Standard target (Foote and MacLennan, 1984). A standard or reference target, normally a sphere which has known acoustic scattering properties, is suspended below the survey transducer. The received echo is a measure of the combined source level and receiver sensitivity. It is now generally accepted that the standard target technique provides the most accurate measurement of the on-axis transducer sensitivity of fishery echo-sounders (Robinson, 1984).

12 3 Techniques for hydroacoustic calibration have developed rapidly in recent years (Foote et al., 1981; Simmonds et al., 1984). Standard calibration targets with well defined acoustic scattering properties have become available, in particular solid spheres of either copper or tungsten carbide cermet (Foote and MacLennan, 1984). The development of techniques for beam pattern measurement has led to more precise estimates of the equivalent beam angle (Simmonds, 1984a). As a result, calibration errors are no longer an important limitation on the accuracy of acoustic stock estimates, provided the calibration is performed competently and in accordance with the procedures described in this report. 1.4 Organisation of the Report In this report, we begin with a discussion of the theoretical background to modern calibration technique. Then we go on to consider the several measurements which comprise the calibration. These are discussed under four headings. The first three, of on-axis sensitivity, time-varied, gain, and equivalent beam angle, are primary measurements in the sense that they are required in calculating the fish density from the echo-integrator reading. The fourth category, electrical measurements, are secondary insofar as they are performed to check on the equipment. Finally, we discuss the accuracy of present calibration technique, and we describe a method of inter-ship calibration. Although the latter method is not a substitute for thefull calibration technique described in the earlier sections, it does allow direct comparison of the complete systems of two or more ships. 2 THEORY AND DEFINITIONS The output of the echo-integrator is used to estimate the quantity of fish per unit area, p, according to the equation p={cg/( V<a>)}M.(1) where C is a calibration factor, g is the time-varied gain (TVG) correction, V is the equivalent beam angle of the transducer, <a> is the average backscattering~ cross section per unit quantity of fish, and M is the echo-integrator output~. The quantity may be either the number or weight of fish. To satisfy equation (1), the same units of fish quantity must be used in the definition of p and <a>. The purpose of the complete equipment calibration is to determine values for the three factors C, g and V. They are defined and discussed in the following paragraphs. 2.1 On-axis Sensitivity The calibration factor C is estimated by integrating the signal from a standard target. If M1 is the echo-integrator output when the. target is on the acoustic axis, then C = a1/(r~m1). (2)

13 -4- where ~Y1 is the effective backscattering cross section of the standard target, as defined by Foote (1982) to take account of the frequency response of the echo-sounder and the bandwidth of the transmitted pulse, and R1 is the target range, na~nely the distance from the target centre to the transducer face or, more strictly, the centre of spherical spreading. The target strength (TS) is related to o in the normal way (Urick, 1975). TS 10 log (o/4ir ) (3) It is important to note that 01 depends upon the pulse duration and echo sounder bandwidth as well as the scattering properties of the standard target p~se. An alternative definition of targnt strength, equivalent to (3), is given by the equation TS = 10 log (Ii/I~). 10 is the incident acoustic intensity at the target and Ii is the backscattered intensity referred to a range 1 m from the target centre. R1 may be measured directly or it may be estimated by timing the received echo. It is necessary to measure the echo time with reference to a precise point on the echo waveform. This might be the start of the echo. However, a more accurate technique is to time the echo at a point where the amplitude is a given proportion of the maximum echo amplitude. The measured time can then be corrected to obtain the time corresponding to the target range. For example, it is important to note that a~ depends upon the pulse duration and echo-sounder, if t1 is the time delay between the start of the transmitter pulse and the half amplitude point on the leading edge of the target echo, then R1 = c(t1 th)/2 (4) where c is the speed of sound, and th is the correction factor which allows inter alia for the signal transmission delay introduced by the electronic filters (MacLennan, 1982). The same TVG which will be used during the survey, namely the 20 log R type, must also be selected during the measurement of M1 and tj. Equation (2) and (4) assume that 1120 log R TVG has been applied, notwithstanding that the signal comes from one target. When t1 is large compared with the pulse length (the target is at long range), the correction th is small. It may then be sufficiently accurate to estimate R1 by measuring the time to the start of the echo pulse, and the small correction may be neglected. 2.2 Time-Varied Gain The receiver amplitude gain is increased in proportion to the TVG function ~(t) where t is the time from the start of the transmitter

14 5 pulse. The factor g in (1) is included to take account of the / deviation ~f d? (t) from the ideal TVG function between the times of the standard target echo and the fish signals. If the fish of interest are within a thin range slice close to range R, and if they are randomly distributed over the cross section of.the acoustic beam, it can be shown (MacLennan and Forbes, 1986) that g = [R exp(br)/~ (R)J2 I [R1exp~R1)/~ (R1)]2 (5) where ~ is the acoustic absorption coefficient expressed in nepers per metre and ~ (R) is the effective TVG function. ~ (R) depends upon (p (t) and its variation over the signal received from a signal target at range R. If the amplitude of this signal is V(R,t), including the effect of TVG, then ~2(R) =Jj V(R,t) Zdt/jI V(R,t)/(p(t) j 2dt (6) Note that ~ and g are functions of range, not of time. The ideal TVG function (p1(t) is such that g = 1 for all R, or 4 1(R) = R exp(~r). This is the so-called 20 log R + 2ci~R form of TVG which is derived by expressing ~ in decibels (db). In the case of real (non ideal) TVG functions, g is estimated as follows. The waveform function V(R,t) is deduced from theory or it may be measured. The calibration procedure includes the measurement of (p(t). In practice, however, the rigorous evaluation of ~ (R) is complicated and an approximate calculation will often suffice. At long ranges, when R is much larger than the pulse length in water, ~ CR) is approximately equal to (p(zr/c). At short ranges, particularly at the range of the standard target when calibrating a transducer in a towed body, it is necessary to take account of system delays. Accordingly, we can write ~(R) = (p(zr/ctg) (7) wh~re tg is a delay, sometimes referred to as the IVG start time. The approximation is to consider tg as a constant independent of R. The delay t~ is a function of the echo-sounder pulse duration and bandwidth. It may be estimated from theory or from an empirical equation, as described later in this report. If (p~(t) is the measured TVG function of the equipment, g is estimated by comparing (p~(t) with the ideal TVG function. For this purpose, a reasonable approximation to the ideal function is one which begins at time tg and then increases in proportion to the time after tg. It is often convenient to calculate g with the aid of a tabulation of the error function E(R) which is defined by: R = ~c(t5 tg) (8a) E(R) = A(p~(t5)/ ~ R exp (~ R) } (8b)

15 6 where t~ is the midpoint of the gate pulse used to sample the receiver output when measuring ~m(t), and A is an arbitrary constant. If R is the range of the fish targets and R1 is the range of the standard target in the on axis sensitivity measurement, then g = E(R1)/E(R) (9) In practice, the range of the fish targets may not be known precisely. It may be known that the fish are in the range interval Ra to Rb, when the average value Er should be substituted for the denominator of (9). This average is simply calculate~ as Er ~ E(R)dR/(RbRa) L.. (10) and g = E(Ri)/Er (11) When working from a graph of E(R), it is convenient to choose A such that Er = 1 over the depth interval Ra to Rb where the fish of interest are expected to be found. Thus A = (RbRa)/ ~ [~m(ts)i { R exp (SR) }JdR (12) 2.3 Equivalent Beam Angle 1 is a measure of the cross section area of the acoustic beam. It is defined by an integral over the echo-sounder beam pattern. See, for example, Simmonds (1984a). I =Jb2 (~)dq (13) cit where b2 () is the combined tra.i~smit-receive intensity response of the transducer in the direction r of the solid angle element d ~l norrnalised to unity on the acoustic axis of the transducer. ii is estimated from measurements of b2(r). If sufficient measurements are available, the integral in (13) is evaluated by summhtg the measurements according to Simpson s rule. Alternatively, the measurements may be used to determine reference points such as the 3 de down points of the beam pattern. Knowledge of,%the theoretical beam pattern may then be used to determine b2( r) at other points and thus to calculatew In the case of narrow beams, say less than 10 between 3 db down points, a small-angle approximation for ~1r may be used (Ona and Vestnes, 1985). 3 ELEMENTS OF CALIBRATION The main purpose of the calibration is to estimate the factors in equation (1) which relate the fish density to the echo-integrator

16 7 output. Three of these factors are considered in sections below. The electrical measurements discussed in section 3.4 are not required for the application of equation (1). However, they are nevertheless an important part of the calibration procedure which must be done at intervals to ensure that the equipment remains within specification, and to detect malfunctions. The particular form of equation (1) is appropriate to calibration by the standard target method in which the source level and voltage response are combined in a single measurement of on axis sensitivity. The standard-target technique is the preferred method for calibrating fishery echo sounders and the only one considered in detail in this report. It is important to note that the standard target backscattering cross section a1 depends upon the bandwidth and other features of the echo-sounder as well as the physical properties of the target itself. In particular, a1 will he altered if the transmitter frequency and the receiver passband are misaligned, although of course it is possible to revise the calibration post cruise. Careful attention to the electrical measurements discussed in section 3.4 will avoid this source of possible error. 3.1 On axis Sensitivity Introduction The purpose of this measurement is to evaluate the on axis performance of the echo-sounder and echo integrator as a complete system. By using a standard target as a known reflector, the combined performance of transmitter, transducer, receiver, and integrator is measured. Thus the transmit signal amplitude, centre frequency and duration, the transducer bandwidth and sensitivity, the receiver bandwidth and gain, and the echo-integrator transfer function are all taken into account. The measurement requires the standard target to be aligned with the acoustic axis of the transducer. Below, two examples are presented which illustrate measurement of the: on-axis sensitivity of the overall system. In the first, the calibration sphere is positioned and then held stationary on the acoustic axis. In the second, the sphere is moved systematically through the central regiqn of the transducer beam and the on-axis response is estimated by interpolation. Tabulated target strength values for recommended calibration spheres are given in Appendix Example: stationary-sphere method Measurement of the on axis sensitivity is performed to determine the calibration factor C in equation (1). This can be accomplished directly by locating ~a standard target on the acoustic axis and integrating the echo. Measurement of the target range R1 and the echo-integrator output M1, together with knowledge of two other system parameters, the TVG correction factor g at the sphere depth and equivalent beam angle 1, allows C tà be determined..

17 Rigging The procedure of the stationary-sphere measurement method is now described. In an example, reference is made to the echo-sounding and echo-integrating equipment used by the Institute of Marine Research, Bergen. The vessel is anchored in calm and sheltered water. The depth must be sufficient for separation of sphere and bottom echoes. It is desirable, moreover, to work in water as deep as possible, consistent with maintaining a stable platform. Both bow and stern anchoring or tying are recommended. This is. illustrated in Figure 1. Winches to guide and steer lines~to the sphere for its centering in the echo-sounder beam are affixed to the deck railing. This is done in accordance with detailed ship drawings. The first winch is placed in the transverse plane of the ship running through the transducer. The second and third winches ar~ placed on the opposite boat side and at equal distances from the transverse section containing the transducer and first winch. Each winch is provided with a long spool of 0.60 mm diameter monofiláment nylon, which is marked with small lead weights at 5 -m intervals, beginning 10 m from the loose end. Prior to commencing the sphere measurements, the lines from the two winches on the same side of the boat are drawn beneath the hull to the other winch by means of a line passed under the keel before anchoring. The appropriate sphere, with affixed loop, is attached to the three suspension lines, cf Figure 1. It is then immersed in a solution of soap and freshwater and lifted overboard by the fastened lines without being touched. The sphere is lowered beneath the vessel to the desired depth, for example, 25 m, which is determined roughly by counting the lead marker-weights on each line. The sphere depth- or range from the transducer is determined by several considerations. The minimal allowable range to the sphere is the greater of the Rayleigh distance, or square of the largest transducer dimension divided by the acoustic wavelength, which defines the nearfield/farfield transition, and the least range for which the sphere echo does not saturate the electronics at the required gain. Two further considerations in choosing the range are the transducer beamwidth and vessel geometry. The physical width of the beam, which increases linearly with range, should be sufficiently great so that the sphere echo is unaffected by the small, perhaps pendular movements to which it is inevitably subjected. The minimal range must also be convenient with respect to the vessel geometry. In particular, if the suspension lines do not hang freely, then control of the sphere may be hindered by friction or possible obstructions on the hull. Despite the number and variety of these considerations, it is seldom difficult in practice to find a suitable range which satisfies all of the above criteria.

18 9- Figiir~ 1. Rigging of a research vessel for stationary sphere calibration.

19 Hydrography Centering During the anchoring and rigging operations, the temperature and salinity profiles should be taken. These will allow computation of the sound speed both at discrete depths and cumulatively to the depths of possible sphere suspension. The second computation will allow determination of the exact depth of eventual sphere suspension from the echo time delay. When this depth is applied in the first computation, the temperature correction to the target strength of the calibration sphere may be obtained from a reference graph or table. The purpose of this crucial opçration is to move the. immersed, suspended sphere onto the acoustic axis of the transducer. Movement of the sphere occurs by turning of the various hand winches, aiwayg oingly and upon specific command by L1i~ director of this procedure, who is guided b~ constant observation ~of the echo waveform on an oscilloscope. The two principles guiding the search for the beam center are Ci) preliminary exploration of the beam to ensure location of the sphere in the mainlobe, and (ii) further probing to find the position of strongest echo. In the case of highly directional transducer.s, determination of the ultimate axial location is made when any movement of any winch, in or. out, cannot increase the echo amplitude Sphere range Measurement of the sphere range~ is necessary for determining the equivalent scatterer density p and the TVG correction factor g in equation (1). The sphere range is determined indirectly by measurement of the echo time delay t and computation of the average sound speed c between transducer and sphere from the measured hydrographic data. The sphere range R1 is then computed as R1 = ct/2. If, for example, the echo time delay were observed to be 30.7 ms and the average sound speed 1490 rn/s, then the sphere range would be 22.9 in. The present method of determining the sphere range is subject to three sources of error. (1) The time delay between the start of transmission, as marked by the trigger pulse, and acoustic response of the transducer is finite. This is a simple consequence of the electromechanical inertia of the transmitting system. (2.) The risetime of the sphere echo out of the background noise and reverberation is finite. (3) The echo time delay t is properly specified through an integral, t = 2 c(z) where z1 and z2 are the respective depths of transducer and sphere, and c(z) is the depth dependent function of sound speed. Use of the average sound speed c in the equation for the sphere range thus involves an approximation.

20 11 These errors, however, are of little significance in big system calibrations, when the transducer-to-sphere distance lies in the typical range m and the signal-to-noise ratio is high. Under such conditions the cumulative effect of the first two error sources is a slight, generally negligible overestimate of the range. The effect of the third source of error is also typically small Echo-integration Integration of the sphere echo links the many individual instruments or processes involved in echo surveying into a single, repeatable operation. The sphere is maintained at its stationary position on the acoustic axis. Three 5 m depth channels are defined in the integrator. The middle of the three exactly straddles the sphere, whose depth is in the middle of the 5 m channel. The other depth channels are placed immediately above and immediately below the sphere channel. These are used to confirm the absence of unwanted echoes which might disturb the sphere measurements. No threshold is used. The echo-sounder and integrator should be set to those operating parameters which will be most often used during the survey. In the case of the Simrad EK38 echo-sounder, for example, these might be the following: transducer 30 x 30 pulse duration external, 0.6 ms TVG ~20 log R attenuator 20 db bandwidth 3 khz range m Simrad integrators require a vessel-speed input. Since the vessel is at rest during the calibration, a constant speed must be simulated. This, might be 10 knots, for example. Given an observation time of six minutes, the simulated sailed distance would be one nautical mile. The integration period, or printout interval, can be set to a smaller distance, but the output values must then be normalized to the average per nautical mi~1e of sailed distance. All correction factors and the calibration constant are equated to unity during this process.. fhat is, neither correction factors nor the instrument constant is applied during the calibration. In this way, all doubt about the values :adopted is avoided. The relative echo energy, or echo energy expressed in the units of the echo-integrator, is computed for each of a large number of pings. The largest of these, if within about 10% of the average, is extracted. If the deviation is greater than 10%, then the centering operation should be repeated and the acoustic measurements performed anew. The largest echo energy finally selected has arisen from a known target and echo-integrator system. Given the relationship of echo-integrator output to backscattering cross section of the standard target, future measurements with the echo integrator may be expressed as absolute fish quantities.,

21 l SL+VR The sum of transmitter source level SL and receiver voltage response VR can be measured while the echo-integrator is being calibrated. Again, the echo derives from a known, on-axis target, and the sonar equation (Urick, 1975) can be solved for the named quantity. It is, in the absence of TVG correction, SL+VRU~~TS~ Z0logR2+2aR2+D1+ZologR1 (14) where U1 is the sphere echo level, TSi is the target strength of the standard target sphere, R2 is the expiration range of the 20 log R TVG function, a is the absorption coefficient used in the TVG function, G is the nominal gain, and Ri is the sphere range. The gain is often described through th~ so-called attenuator setting. It should be noted that a positive! attenuation is equivalent to a negative gain of the same magnitude. The units of the several quantities are shown in the fol1ow~ng table. The reference voltages and pressures may be either root-mean-square (rms) or ~ak-to-peak values. However, consistent use of rms or peak-to-peak~is essential to ensure that equation (14) is satisfied. TABLE Quantity Symbol Units Source level SL db 1/ 1 ppa at 1 m Voltage response VR db 1/ 1 V per jipa Echo Voltage level U1, U2 db 1/ 1 V Target strength TS db Ranges R, R1, R2 m Absorption coefficient a db/m Attenuator setting db The measured output quantity is the peak or rms echo amplitude. For a constant-amplitude sinusoidal signal, the rms value is the peak amplitude divided by 2~. Correction of the equation for SL + VR for a possible deviation in TVG at the sphere depth is straightforward. If, for example, the first determination of SL + VR yields db, and the TVG is 0.3 db too high at the sphere range, then the correct value for SL + VR is db.

22 Example: moving-sphere method Method A system has been developed for moving a standard target automatically through the transducer beam, and computing the onaxis sensitivity by interpolation. The present system has been designed for use with towed bodies, but a scaled up version could be used with a hull-mounted transducer. Experiments with the stationary-sphere technique reveal that a ball hung on monofilament nylon is liable to move. Results over a period of hours may be quite variable. This seems to be due to three main effects: ship movement, water currents and water absorption in the monofilament nylon which alters the twine length. An alternative technique is to move the sphere successively to a number of positions in a scan through a plane section of the acoustic beam. A curve is fitted to the echo-integrals recorded at each position. The procedure is repeated for a second scan in a section at right angles to the first, and including the maximum of the first fitted curve. Tho curve maxima rapidly converge to give a consistent estimate of the on axis sensitivity. In practice, the echo-integrals might be accumulated over 30 transmissions at each position, and the curve might be fitted to 11 points per scan. In ideal conditions, a series of 80 such scans produced results within ±1%. However, when the same quantity of data was collected with the sphere stationary at the apparent beam centre, the results covered a range of more than ±5%. The scanning technique uses the curved beam pattern to best advantage and increases the precision considerably. It is therefore recommended that wherever practicable, a scanning method should be used. The measurement is performed~ using a standard target (38.1 mm diameter tungsten carbide sphere) suspended on three strands of monofilament nylon each attached to an adjuster placed at the end of an arm positioned above the transducer and towed body (Figs 2 and~ 3). The construction! of the nylon twine container for the target is illustrated in Figure 4. Encoders on the motor output shafts allow the twine lengths to be displayed. The adjusters may be controlled manually or by comput~er. The output of the echo-sounder is connected to an integrator or sample gate which should be set to include the complete echo. The adjusters may be controlled to move the sphere in two independent planes. For the system used at the Aberdeen Marine Laboratory, the sphere is hung on 5.5 m lengths of nylon and the three arms are arranged radially so that the suspension points are 3 m apart. The adjusters provide ±100 mm of twine movement to an accuracy of 0.33 mm. The full extent of adjustment is used for the first scan. A series of 21 points is selected by adjusting the length of one twine in 10 mm steps. Thirty transmissions are carried out at each point and the total echo-integral recorded. These data are then used to compute a parabola by least squares fit. The curve is fitted to the 4th root of the data in order to reduce errors due to the non-parabolic shape of the beam pattern. The maximum value and the position of the

23 -14-- Figure Z. method. Rig and target suspension for towed-body calibration: moving sphere rotation encoder reed switches upper imit switch travelling piston limit switch Figure 3. Mechanism for adjusting the length of twine supporting the standard target. Three such adjusters are shown at the arm ends in Figure 2.

24 15 == (a) ic) (d) (e) ~Z~68710 Figure 4. Construction of a monofilament container for standard target spheres. The numbers show the order of tying knots. Successive knots are one quarter of the sphere circumference apart.

25 16 maximum are computed from the equation for the parabola and the adjuster is moved to this position. Then the second scan is performed by adjusting the other two twines, one is lengthened and the other shortened, to move the sphere in a section at right angles to that of the first scan. The curve fitting procedure is repeated to determine the maximum echo-integral and the corresponding position. The adjusters are moved to the computed position, then the first section is scanned again. Following the first two scans, only 60 mm of adjustment and 13 points per scan are required. For each completed scan a maximum echo-integral and the corresponding position are computed. The maxima rapidly converge upwards and under good conditions will vary by ±1% only. This dynamic measurement procedure produces significantly, superior results to the stationary-spher~ technique used with the same rig Sphere range In order Lu ~akulale the oii axi~ sensitivity of the ec~ io-sounder, the range to the standard target must be measured. Thi~ could be done directly, by tape measure, but a more practical and accurate alternative is to measure the time of the echo from the target. The half voltage point on the leading edge of the received echo is a well defined reference time. This can be measured by examining amplitude samples from the echo, if such are available, or by triggering the oscilloscope with a delayed pulse so that the echo waveform near the half, voltage point is visible on an expanded scale. The time between the transmission and the delayed trigger pulse may be accurately measured using a counter timer. Measurement of the time delay by visual examination of the oscilloscope trace is inaccurate and should be avoided if at all possible. To calculate the range of the target, the system delay. th due to bandwidth must be subtracted from the measured echo time. Tables 1(a) and 1(b) show values of the system delay appropriate to copper and tungsten carbide calibration spheres. The value of th is selected according to the echo-sounder parameters being used, and the range R1 of the target may now be computed from equation (4) TVd correction There is an additional correction required which corrects for TVG errors at that point of the TVG where the calibration is carried out. If E1 is the echo integrator reading, then Ml = E1/g(R1). The cbrrection factor g is obtained from Table 2, corresponding to the target range R1, and the calibration factor C may now be calculated from equation (2) Worked examples Echo-sounder EK400, 38 khz Transducer 34 elements, beamwidth 8 x 13 (3 db down points) Pulse duration 1.0 ms Bandwidth 3.3 khz TVG 20 log R Control settings High output power; receiver gain -10 db

26 17 TABLE1 Echo-timing Data for Target Range Calculation (a) Echo-sounder: Simrad EK400 Target: 60 mm copper sphere Bandwidth 1 khz 1 khz 3 khz 3 khz Pulse duration 1 ms 3 ins 1 ms 3 ms Half-peak time ti (ins) Signal delay th (ins) infinity (b) Echo-sounder: Simrad EK400 Target: 38.1 mm tungsten carbide sphere Bandwidth 1 khz 1 khz 3 khz 3 khz Pulse duration 1 ins 3 ins 1 ins 3 ms Half-peak time t1 (ins) Signal delay th (ms) infinity NOTES The target range R1, from the target centre to the transducer centre of spreading, is estimated as R1 = c (ti th)/2, where c is the speed of sound, and t1 is the time from the beginning of the transmitter pulse to the point on the receiver output waveform at half the peak amplitude, with time varied gain applied. The, signal delay th has been calculated for a receiver with 20 log R time-varied gain.

27 18 TABLE 2 Data from TVG Measurement ~ Measured Time of. Range of Theoretical Sample Optimised Error input sample sample TVG inter- measured function val TVG U1 t~ R~ 41(R) W~ ~0,~(R) g(r) (mv) (ms) (m) (iii) ~ ~ k S ,~ ~ ~ , ,

28 19 TABLE 3 Estimation of the On-axis Sensitivity by Moving the Target Through the Transducer Beam Scan 1 (Forward Adjusters) Encoder Echo Fitted position integral parabola (V2 ms) (V2 ms) Scan 2 (Rear Adjusters) Encoder Echo Fitted position integral parabola (V2 ms) (V2 ms) Z ~ Z9-~.1-ZOO Solve for Maximum

29 20- Table 3 shows the standard target echo integral against the encoder position, from the 11 points of the first scan. The base of the Table shows the maximum value estimated from a least squares parabolic fit to the fourth root of the data, and the right hand columns show similar data for the second scan. Again the maximum value is estimated by curve fitting. In the second scan, although adjusters 2 and 3 are moved together, only 2 is used to define the abscissa and 3 is assumed always to remain in step. These two scans, moving the sphere in mutually perpendicular planes, are used to define the position of the centre of the beam and the corresponding echo integral E1. To locate the beam centre, a pair of scans over the full range of the adjusters are carried out first. ~These data are not used to compute a maximum echo integra1~ The curve-fitting procedure underestimates the maximum echo integral because the parabola is an approximation to the true beam shape. In order to reduce this bias, subsequent scans are carr4ed out over a sma1le~pection of the beam, adjusting the nylon twins length by only ±3 cm~ The error due to the parabolic approximation is now less than 0.1% and can be ignored. To correct for the range of the target, the time to the half voltage point of the echo waveform is measured. This will be nearly independent of movement in the second scan, when two adjusters are moved together (in opposite directions). However, the range does change significantly during the first scan, when one adjuster is moved onl its own. In addition, the nylon can absorb water and stretch, thus altering the target range during the calibration. Therefore, it is important to measure the range frequently, near the times of echo amplitude measurement. The estimated value of the on-axis edho integral is the mean result from, pairs of scans. Four or five pairs of scans should be performed and their results averaged, after omitting any spurious values. The time-delay to the half voltage point is measured as t ins. The system delay th from Table 1(b) is 0.46 ms. Ej from Table 2 is V2 ins but, from Table 2, it is subject to a TVG correction factor of The sound speed c, estimated from hydrographic data, is 1490 ms. Hence = 1.49 ( )12 = 6.47 m = /1.009 = V2 ms The target strength of the 38.1 mm tungsten carbide sphere, for c 1490 ms and 1 ins pulse duration, is db. Hence a cm2 and, from equation (2) C = x 10 4/(6.472 x ) = (V2 ms)~

30 Z Time-Varied Gain Introduction The time-varied-gain (TVG) function of the typical echo-sounder generally deviates measurably and not insignificantly from the nominal or ideal specification. This means that echoes from fish at different ranges may be compensated inaccurately, thereby biasing estimates of fish density with respect to depth. This biasing may occur irrespective of whether the TVG is correct at the depth of the standard target during the calibration. It is therefore necessary to measure the actual gain of the echo-sounder over the range of applicability. This range generally extends from the start of the TVG to the so-called expiration range, when the receiver gain attains, and remains at, maximum value. The TVG correction factor is determined by comparing the observed TVG with the ideal or desired gain over the entire compensation range. Specifically, the factor g in equation (1) is determined as a function of range by computing the ratio of measured and ideal gains as a function of time. Measurement of the TVG generally requires instrumentation external to the echo sounding and echo-integrating equipment. Signals are applied to the echo-sounder input, and the corresponding receiver output is measured. The estimation of the ideal TVG function and the TVG error depends upon knowledge of the sound speed (c) and acoustic absorption coefficient (A or ci). Both parameters, depend upon hydrographic factors such as temperature, salinity ánd depth (Foote, 1981). Error in the values assumed for sound speed and absorption leads to error in the TVG function and hence to bias in the fish density estimate. It is usually most convenient to estimate c and S (or cl) as functions of the hydrographic parameters, using empirical equations. Many different equations will be found in the literature, but those which are~urrently recommended for acoustic survey purposes are shown in Ap~endix I. It is important that adequate hydrographic information is obtained relevant to the area surveyed, to allow the sound speed and absorption coefficient to be estimated. To some extent, temperature and salinity data may be obtained from charts or other literature, but it is recommended that temperature - salinity depth (TSD) data be collected at intervals during the survey, including any anchorage where calibration is performed. There are two basic approaches to the problem of TVG measurement. In one, the input signal is held constant, and the echo-sounder output signal is measured. A disadvantage of this approach is that in order to avoid saturation at the expiration of the TVG function, the output signal. must be very weak initially. In the second approach, the output voltage is held constant, and the input

31 -22- signal is varied. The disadvantage with this method is that the input voltage must be very large at the beginning of the TVG ramp, which can result in the amplifier saturating. This. may be avoided by careful choice of signal level and echo sounder repetition rates. The twa methods are now described. In the first example, illustrating the constant output method, the measurement is effected by widely available laboratory instruments. In the second method, adapted from the constant-input technique, the measurement is performed by a specially-built instrument, the Time-Amplitude-Frequency (TAF) Unit (Knudsen, 1985) Example: constant-output method Method This method has several advantages. It may be carried out automatically or manually; it requires standard commercially available equipment, and relies ~n1y on the calibration of a single voltmeter for its accuracy. OneI limitation is that th&putput level must be chosen carefully, so as to avoid saturating~the receiver amplifiers while short range values are being measured. It helps to run the echo sounder at the fastest repetition rate possible for any given measurement point. A signal source is connected to the input of the echo sounder (test input). A sampling gate, or an echo-integrator which serves the same function, is connected ~to the. output of the echo-sounder (calibrated output). An AC voltmeter is also connected to the echo sounder input unless the signal source is programmable and accurate. The sampling gate width is set equal to the transmitter pulse duration to be used on the survey (or as similar as practicable). The sampling gate is set to the first range point. The time to the start of the sampling gate must be known or measured. The signal source is then adjusted to give the chosen output from the sampling gate. The input level is then recorded along with the gate time measurement. The gate is moved to the next range point and the input level adjusted to give the chosen output again. The time measurement and input level are recorded, and further measurements repeated over the full range of the TVG function. An. automated measurement procedure may be based on a synthesised signal generator (Fluke 6011 or equivalent) as the signal source. A crystal controlled range gate is used to sample the echo sounder output and the gate output is read by a computer. The signal generator may be programmed to control the gate output to within ±1%. The gain.is then the gate output divided by the programmed voltage input to the echo-sounder. A series of transmissions may be averaged to improve accuracy. In the case of the EK400 at 38 khz, for example, take the average of 10 transmissions at range points up to 300 m, and 40 transmissions at greater ranges. A total of 50 range points are required to give ±1% precision for the TVG correction factor g.

32 Calculation of the TVG error function The measurements discussed above give a set of times t~, from the start of the transmit pulse to the mid.point of the sampling gate, and a set of corresponding input voltages, U1. Suppose the ideal TVG is ~~(R), where R = R~ is the nominal range corresponding to the time t1. For 20 log R + 2 c~r as the ideal TVG function in db: = c (t~ tg) (15) ~1(R) = R~2 (102a Ri/b). (16) ~ m,i(r) = (1/u~)2 where ~ m,i(r) is the measured TVG, t~ is the time at which the measurement of the echo-sounder output is made, c is the sound speed appropriate to the hydrographic rrrnditirrns expected during the survey, tg is the TVG start time, see Table 4, and ~L is the acoustic absorption coefficient in db m1. The measured gain may be multiplied by a constant C~ to minimise the error over any given range interval. The minimum error TVG function is: ~ 0,1(R) = ~ m,i(1?~) C~ = (1/U~)2 C~ (17) where C5 = (Ra Rb) ~ ~i ~ m,i (R)/~I j (R) (18) W~ is the sample interval, equal to (R~~1 R1_1)/2. The range interval Ra to Rb is selected to include the depths at which the fish of interest are expected to be found. The sum is taken over all readings for which Ra<Rj<Rb. The values of ~ 0,~(R) may then be computed as described aboye. This provides an error function g(r) = ~ o,(~if~ ~(R) which has been minimised fnr the depth range of interest but which indicates the magnitude of the TVG error at other ranges also. The error function may now be used to calculate the error at the range of the standard target during the calibration. Any integrator measurement may be coi~rected by dividing the energy integral by the error function g(r) for the appropriate range Worked example Echo-sounder: EK400, 38 khz Constant output level: 0.5 V peak-to-peak This output level is about the middle of the 50 db dynamic range of the echo-sounder. It is also small enough not to overload the receiver, provided the echo-sounder repetition rate is kept as fast as possible. The sample gate width is set to 1 ms, equal. to the pulse duration. The measurements of input U~ and time t~ are shown in columns 1 and 2 of Table 2.

33 -24-- TABLE 4 Ideal TVG Start Time for Exact Range Compensation (a) Echo-sounder: EK400 Target: 60 mm copper sphere Bandwidth I khz 3 khz Pulse duration 1 ms 3 ms 1 ms 3 ms Echo half peak t1 (ms) TVG start time tg (ms) infinity (b) Echo-sounder: EK400 Target: 38.1 mm tungsten carbide sphere Bandwidth 1 khz 3 khz Pulse duration 1 ms 3 ms 1 ms 3 ms Echo half peak t1 (ms) TVG start time tg (ms) infinity

34 -25 For a 3 khz bandwidth at 1.0 ms pulse duration tg = 0.92 ms (Table 4b). The calculated R1 for c = 1490 ms are shown in column 3 of Table 2. The theoretical TVG in db is 20 log R + 2 ar, with a = db m. The TVG has been optimised for its full range. From equations (15-18), C5 is calculated as.0781z, for Ra = 2.5 and Rb = 529 m. Column 7 of Table 2 gives g(r), the error for a target at ranger=r1. From this it is seen that errors are typically less than ±3%. If a standard target calibration were carried out at 6.5 m range, the error there would be +0.9%. Thus the echo energy recorded during the standard target calibration must be corrected by dividing by This is the factor g(r~) used to calculate M1 (cf section 3.1.3). The factor g in equation (1) is the mean value of g(r) over the depth range of interest. If all depths are equally of interest, then g=1 by virtue of the optimisation. 3,2,3 Example: measurement by a special-purpose unit Until recently, measurement of TVG has generally been accomplished by standard instruments. These typically include a signal generator, frequency counter, voltmeter, and oscilloscope. The value of a special electronics unit which can measure the TVG function without requiring a large number of separate instruments is thus evident. The resultant simplification may also increase the reliability of the measurement. The following example refers to Simrad echo-sounding and echo integration equipment. Full details of the design and operation of the special purpose unit will be found in Knudsen (1985) The Time-amplitude-frequency (TAF) unit In order to simplify the pi~ocedure, a specially constructed test generator is used. This controls the time, amplitude and frequency, henc~the acronym TAF. T~he test generator produres a number of pulse~ generally 20, which simulate fixed depths. In this manner it is sufficient to know the pulse number, without having to measure the time. The amplitude is regulated in two steps. The first is high for the period corresponding to 100 m range. For the remaining TVG range, the amplitude is reduced by. 20 db. This ensures both a sufficiently strong signal at the beginning of the TVG range and avoidance of saturation in the preamplifier at greater ranges. The frequency of the TAF unit is determined by a crystal. Adjustment of the frequency is therefore unnecessary Measurement of the TVG deviation with TAF It is assumed in the following that reading of the TAF instrument occurs manually by means of an oscilloscope.

35 26- (1) The TRIGGER signal from the echo-sounder is connected to the input port TRIGGER IN. (2) The SIGNAL OUT is connected to the TEST INPUT of the attenuator, which is internal in both the EK and EK-S models. (3) The echo-sounder is run with 250 m main range, and the 3 khz bandwidth is selected. (4) The TRIGGER OUT signal is connected to the EXTERNAL TRIGGER on the oscilloscope. (5) Pulse number 20 is selected, and the CAL OUT signal is adjusted to 10 vp_p.. Pulse number 20 is well outside the TVG range, hence the receiv~r has reached its maximum amplification. The amplitndes of th~ prereding p~i1ses are now cletermh~ed relative tothelast Worked example Consider computation of the amplitude of pulse number 2, given that the amplitude of pulse number 20 is 10 ~ or 11 db re 1 V rms, for the EK400/38. Pulse number 2 simulates 20 m depth. At depths less than 100 m, the signal generator output is 20 db. higher and the computation takes this into account. U20 m = Umax (20 log R + ZctR) + (20 log x cx x 20) + 20 = = db or 1.23 vp_p where R = 581 ni approximates the cutoff range of the 20 log R TVG function for this echo sounder, and cx = db/m is the absorption coefficient. The quantity U20 m is the theoretical value for the voltage, which can he nompared with that read off the oscilloscope. Tables 5 and 6 show the computed values for the EK38 and EK400/38, respectively. 3.3 Equivalent Beam Angle Introduction A transducer radiates and receives energy with different sensitivity according to the target position in the surrounding volume. In order to use a transducer for fish surveys, it is necessary to know the total energy transmitted and received. Normally this is determined by the on-axis sensitivity and a measure of the beamwidth which is called the equivalent beam angle. The latter has been defined in equation (13). A theoretical value of I may be calculated for any transducer. However, experience has shown that precise measurement is necessary. Whether or not the equivalent beam angle is specified by

36 27 TABLE 5 Theoretical voltage amplitude Ur at range r for the calibrated output signal of the EK3S echo-sounder, based on the maximum output signal amplitude Um~ = 10 ~ at the expiration range R = m, given that c~.= db/m. Note that the signal is reduced by 20 db for ranges greater than 100 m. Pulse Depth Voltage amplitude U1. number (m) (db) (V~_~) CUr/Umax) TABLE 6 Theoretical voltage amplitude Ur at range r for the calibrated output signal of the EK400/38 echo-sounder, based on the maximum output signal amplitude Umax 10 Vp_n at the expiration range R = m, given that c~ = db/m. Note that the. signal is reduced by 20 db for ranges greater than 100 m. Pulse Depth Voltage amplitude U1.. tpimber Cm) (db) (V~_~) Z

37 28- the manufacturer, it should be measured by the user at least once after mounting the transducer. This is important both for hull- and towed-body-mountings, and the type of mounting is likely to modify the beamwidth (Simmonds, 1984b). Only one method for measuring V is presently fully developed. This method is primarily suited to towed-body systems and requires a special rig. However recent developments (Ona and Vestnes, 1985; Reynisson, 1985) suggest other methods suitable for vessels with hull-rn ounted transducers. To a first approximation, the equivalent beam angle changes in proportion to the square of the sound speed. It is therefore important to note the sound speed when the equivalent beam angle is measured Example: towed-body transducer Method The measurements are carrieã. out in Loch Duich ofi the west coast of Scotland. The underwater equipment is suspended below a raft which is moored about 600 m from the shore. The raft is connected by cable to a cabin on the shore. The transducers are Simrad type 38-26/22-E, constructed from 34 elements resonant at 38 khz, arranged in a rectangular pattern. The beam dimensions are assymetrical, nominally 8 by 13 degrees between the 3 db down points. The transducer is placed at the centre of a motorised gimbal table supported by a triangular frame suspended on three 15 m wires at a depth of 20 m below the raft. A 38.1 mm diameter tungsten carbide sphere is hung 5 in below the triangular frame by three strands of monofilament nylon. The rotation of the gimbals is determined using digital angle encoders. The angular information is transmitted by a serial rommunirations link to the shore, and the drive motoro for the gimbals are remotely controlled over the same link Data, collection A Computer Automation 4/90 computer is used to control the gimbal table and the transmitter. It also performs a preliminary analysis of the received echo data. The 38 khz transmitter provides a 0.5 ms pulse. The frequency is crystal controlled. The receiver is a switched gain amplifier with 7 gain steps of 3 de and a single tuned filter of 4 khz bandwidth. The signal is envelope-detected and sampled every 100 ps. Each sample is converted to a 12 bit binary number and passed to the computer. The samples are squared and summed to give an estimate of energy within the returned echo. The system provides 50 db of dynamic range with an accuracy of ±0.1 db and a further 25 db with the same linearity but lower precision. In practice, only 30 db of dynamic range is required to provide acceptable results.

38 29 The measurement of the transducer beam pattern is performed by recording integrals of the energy in the echo from a sphere suspended below the triangular frame. The transducer is used both as projector and receiver, as in the echo sounder, to evaluate the combined response. Data are collected at 0.2 intervals along ±15 scans in one direction, that of the narrower beamwidth, and at 0.5 intervals over ±150 in the perpendicular direction. The sections of the hemisphere from which data are collected are restricted to ±15 movements of the gimbal in order to save time. Errors caused by this restriction are negligible, about 0.01 db. At each point, 40 transmissions are carried out and the standard deviation computed. Data are accepted only if the standard deviation of a set of 40 echo-integrals is less than 2% for signals down to about 15 db below the on-axis level, 8% for the next 20 db down and 40% for the remainder. 3.3.Z.3 Data processing The data from the grid of 61 by 151 points are inspected and any obviously spurious values replaced by linear interpolation from adjacent points. This correction procedure has a negligible effect upon the final results (less than db). The individual values are multiplied by M~, the element of solid angle associated with each point, and summed. The data are processed to give the following estimate of the equivalent beam angle. 1 = Z b2 ~ Af1/b?~rriax (19) where b2max is the estimated on-axis sensitivity Example: hull-mounted transducer Principle Materials The following description is derived from Reynisson (1985). The&beam pattern of thel transducer is sensed by moving a target sphere systematically alkrng arcs in each of several different vertical planes passing through the acoustic axis. The equivalent beam angle is computed in accordance with its definition in equation (13). The vessel is rigged similarly to that for measurement of on-axis sensitivity by the stationary-sphere method, cf section and Figure 1. The arms holding the sphere suspension lines away from the hull may have to be extended to avoid contact between the suspension lines and thehuli. The suspension lines should be made of stainless steel wire or other similarly stiff material. Distances along the lines should be accurately marked at intervals of 1 m. Use of a meter rod beside each winch, cf Figure 5, will aid the positioning procedure.

39 30 Figure 5. Equipment for transducer beam pattern measurement. The meter rod is used for precise length control of one of the lines suspending the sphere.

40 Method If the target sphere is relatively light, then weight suspended1 beneath it will add inertia, hence stability, to the suspension system. The distance between the stabilizing weight and the target sphere should be large enough so that the two echoes do not overlap. To begin each measurement series, the target sphere is centred in the transducer beam. This may be done as described in section The lengths of the three suspension lines, from the common attachment point on or near the net bag to the points of first contact with the winch system, are noted. The energy contained in the sphere echo is measured for each of 100 echo-sounder transmissions, and the arithmetic average energy is computed Analysis The length of one of the suspension lines is then increased or decreased in small steps. For calibration of typical ocean-going research vessels, with calibration sphere ranges of m, an increment of 5-10 cm is convenient. At each new step or distance the measurements of echo energy are repeated. The length of the suspension line under adjustment is increased or decreased until the echo strength has fallen by at least 3 db with respect to the on-axis value. The length is then changed systematically in the opposite direction, pulling the sphere through and past the acoustic axis until the -30 db level is again reached. The sphere is returned to the acoustic axis. Another suspension line is now shortened or lengthened in the same manner as the first, and the measurements repeated as before. The procedure is finally repeated for the third suspension line. In describing the beam pattern, the measurements of echo energy are compensated for the precise distance of the sphere from the transducer at each measurement step. This distance is determined by geometry. Aii~example of measur~ment results for the 38 khz transducer mounted on the hull of ~jarni Saemundsson is shown in Figure 6. Equal-energy contours derived from these data are presented in Figure 7. The data illustrated in Figures 6 and 7 are used to determine the equivalent beam angle, by a approximate calculation based on equation (13). The result for this particular transducer is V = sr (10 log P = 20.3 db). It is interesting to compare this value with that determined from the beam pattern published by the manufacturer, pertaining to a similar transducer before mounting, cf Figure 8. This is sr (-18.8 de). If the nominal value specified by the manufacturer for the unmounted transducer were to be used, then the fish density would be underestimated by 29%, since I occurs in the denominator of

41 log b2 (db) B Log b2 1dB) Log b2 1dB) ~ ~2i,1~~1U DEGREES Figure 6. Measured two-way directivity patterns of the 38 khz hull-mounted transducer on Bjarni Saemundsson.

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43 -34- equation (1). To avoid this source of bias, measurement of the transducer beam pattern after mounting is recommended. 3.4 Electrical Measurements In order to monitor system performance, a number of parameters not directly involved in equation (1) ought to be regularly and frequently measured. These include characteristics of transmitter, transducer, receiver, and echo-integrator. Such measurements are often specific to particular instruments. The examples worked below apply to Simrad equipment Transmitter It is important to check the transmitter output power. For pulsed echo-sounders, the power is normally determined by means of an oscilloscope used with either a voltage probe or current probe. Both methods are now considered for the case of a 1 ms pulse applied to the transducer Power measurement with a voltage probe Example A 100 x probe that is properly compensated for high frequencies should be used. This is connected to the terminals of the subject transducer. Exactly where this is done depends on the particular equipment that is to be measured and how it is mounted. What is important is that the voltage that drives the transducer be measured. Figure 9a shows an oscilloscope photograph of a transmitter pulse measured with a voltage probe. There are often irregularities at the beginning and end of the pulse. These frequently take the form of overshoots or tails. They have only a slight effect with respect to the energy contained in the total pulse, but should the voltage be read off at an irregularity, large errors may result. The voltage of a 1 ms pulse should be read 0.5 ms after its start. The voltage of a 1 ms pulse is determined from an oscilloscope to be 1250 vp_p (peak to peak) 0.5 ms after the start. The impedance Z3 at the nominal centre frequency of the transducer was found earlier to be 63 Q. The average power is thus P =u2~~/(8 z3) = 12502/504 = 3100W where denotes the peak-to-peak voltage. According to the manufacfurer s specifications, the transmitting power response S~ is db//l ppa at 1 m per W. The expected source level is thus SL = S~ + 10 log P log db//1~tpa at 1 m The method in this example must be considered unreliable because the phase relationship between voltage and current is unknown, and

44 35 (a) (b) Figure 9. Oscillographs of a 1 ms transmitter pulse measured (a) with a voltage probe; 100 x amplification; scales: 500 V/cm vertical, 0.2 ms/cm horizontal; and (b) with a current probe, conversion factor 10 ma/mv; scales: 10 A/cm vertical, 0.2 ms/cm horizontal.